Voice coil motors (VCM) are used in many applications. They are substantially composed of a winding placed in a magnetic field generated by a permanent magnet By forcing a certain current in the windings, a force that displaces the winding is generated. The displacement of the winding may be controlled with high precision.
Besides VCMs, there are other electro-mechanical actuators that work by exploiting this principle, such as audio loudspeakers, electro-locks, etc. Because of the wide use of VCMs, reference will be made to this class of motors, but what will be stated holds for any voice-coil like actuator
VCMs are largely used for moving the arm that supports the read/write heads over the rotating disk of a hard drive, and to move it to and from a parking ramp.
The ramp unloading (parking) operation of the read/write heads is essential for preventing possible impacts with the hard disk during transportation. In fact, if the mechanical arm is subjected to strong vibrations, it could flex and the read/write heads could hit the hard disk damaging it or themselves, if not safely parked. The reverse operation, called ramp loading, is performed each time the hard disk is turned on for reading from or writing data in it.
It is important that the swing (displacement) of the mechanical arm be controlled during these operations for preventing possible damage to the heads when the arm reaches the stop position on the parking ramp, or when bringing the heads onto the disk.
A dedicated control circuit controls the translation speed of the mechanical arm moved by the VCM motor during ramp loading and unloading operations. According to a known technique for controlling the speed of a VCM, the speed of the mechanical arm is measured by optical encoders, as disclosed in the U.S. Pat. No. 5,455,723.
The speed of the mechanical arm may also be measured by sensing the back electromotive force (BEMF) induced in the motor winding by the arm motion The back electromotive force that is generated by the arm motion is generally proportional to the translation speed of the arm. This back electromotive force may be measured by sensing the voltage on the nodes of the motor winding when the motor winding is in a high impedance state (tristate), as disclosed in U.S. Pat. No. 6,542,324.
In a discontinuous functioning mode, the output stage that powers the motor alternates conduction phases, during which the motor is substantially connected to the supply line and off-phases during which the motor is placed in a high impedance state (tristated). With TON being the duration of a generic conduction state and TOFF being the duration of a generic period during which the motor is tristated, no current circulates in the winding for a time TOFF and the voltage measured at the terminals of the motor is theoretically equal to the back electromotive force (BEMF)
A control system of a VCO that implements this method is shown in FIG. 1 It allows control of the motor either in a voltage mode or in a current mode. Control circuitry, not depicted in the figure, controls the switches T1, T2, T3 and T1 of the power stage through control signals The operational amplifier senses the back electromotive force BEMF induced in the winding of the VCM by sensing the voltage at its terminals when all the switches of the power stage are open.
A drawback of this technique is that it is necessary to wait for a minimum time TOFFMIN before the voltage at the terminals approximates with sufficient precision the induced back electromotive force BEMF. Tests carried out on a VCM showed that, from the instant in which the motor is tristated, there is a transient component of the voltage on the motor that could affect the sensing of the BEMF.
This effect is illustrated by the waveforms of FIGS. 2 and 3, which show time graphs of the currents and of the voltages on the nodes OUT_P and OUT_M during the intervals TON and TOFF, in two different driving conditions of the VCM. The scales of the current and of the voltage are respectively 200 mA/div and 5 V/div, while the time-scales are 50 μs/div and 10 μs/div for the graphs of FIGS. 2 and 3, respectively.
Even when the current transient has decayed, there persists a voltage transient that typically lasts several tenths of microseconds longer, and that may alter the sensing of the BEMF. By comparing the graphs of FIGS. 2 and 3, the amplitude of this prolonged voltage transient depends upon the current that has flown through the winding during the preceding conduction phase.
To sense correctly the BEMF induced in the winding of the VCM, it is necessary that the motor be tristated for a time TOFFMIN sufficiently long to let the transient voltage decay. These phenomena limit the maximum switching frequency of the control signals of the switches of the driving stage, and as a consequence, the precision of control of the speed of the motor. This maximum switching frequency typically ranges between 1 kHz and 3 kHz, thus acoustic noise is generated.